Viscous fluid couplings have received wide acceptance in the automotive industry for controlling the amount of torque transmitted to a radiator cooling fan. The most common form of such viscous fluid couplings is the air temperature response type such as illustrated in U.S. Pat. No. 3,055,473. In certain applications, however, it has become desirable to directly sense the engine coolant temperature rather than temperature of the air passing through the radiator. To date, many arrangements have been proposed to accomplish this result. Typically, these arrangements have made use of wet or dry plate clutches that are electrically actuated. In both of these arrangements, the advantages achieved through the use of viscous fluid have not been applied.
The need for a fan drive having more than one speed ratio is evidenced by U.S. Pat. No. 3,444,748, which illustrates a fan drive including a fluid coupling providing a high speed ratio (i.e. ratio of output or fan speed to input or engine speed) at low engine speed when relatively little air is being forced through the radiator due to movement of the vehicle, and an overrunning clutch for providing lower ratios at high engine speeds so that less horse power is being wasted driving the fan when higher vehicle speed is forcing a greater amount of air through the radiator. Reducing the speed ratio of the fan drive at higher engine speeds is also desirable to reduce the heat built up within the fan drive. However, such fan drives often include no provision for disengagement when the air temperature within the engine compartment is such that operation of the fan is unnecessary. Furthermore, such fan drives cannot be modified in an obvious manner to provide for purely temperature-responsive multispeed operation when a higher speed ratio at higher temperature is required.
Although temperature responsive multispeed viscous couplings have been proposed in the prior art, they have generally not been widely commercially successful due to their complexity, size and cost. One approach to dual speed operation has been to provide dual input members with associated mechanical drive mechanisms which are selectively engaged to drive the input shaft of the coupling at different speeds. Another prior art approach to multispeed operation has been through the restriction of fluid flow through a relatively large drain port by an overlaying valving element which is repositioned to provide, in effect, a variable orifice. Such approaches, although gaining some commercial acceptance, have proven to provide poor repeatability by being unable to precisely control the amount of viscous fluid in the operating chamber of the fluid coupling. Additionally, such devices typically repositioned their valve by the use of a single temperature sensing element which, although operating well about a single calibration point, was unable to provide reliable operation at two or more varying temperature set points or over a wide range of operation.
Prior art temperature responsive multispeed devices often display the attribute of "hunting", i.e. repeatedly shifting between speeds in a step fashion, and thereby generating undesirable abrupt engine load changes and subjectively annoying high noise levels. Additionally, many applications otherwise well suited for a simple "on-off" type device, require multispeed operation only during transitional operating modes such as short term cycling. Such applications have generally been filled by true multispeed prior art devices which are relatively complex and expensive.
It will be apparent from a reading of the specification that the present invention may be advantageously utilized with fluid couplings intended for many different applications. However, the invention is especially useful when applied to a viscous coupling or clutch which serves as a drive for a radiator cooling fan of a vehicle engine, and will be described in connection therewith.